About this replacement
Xiaomi Mi Note / Libra — 3.8V Li-Polymer Replacement Battery (BM21)
This is a 3.8V, 3000mAh (11.4Wh) lithium-polymer cell built to replace the original BM21 battery in the Xiaomi Mi Note and Mi Note Libra smartphones. It fits the standard battery bay without modification. Install it when the original cell no longer holds a useful charge or causes unexpected shutdowns.
- Mi Note and Libra compatibility: Both the Mi Note and Libra variants use the same BM21 form factor — identical footprint at 78.90 × 62.64 × 3.38mm, same connector pinout, and the same 3.8V nominal voltage rail the PMIC expects. No adapter or modification needed.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on Mi Note hardware. The BMS accepted charging without tripping overcurrent protection, and the fuel gauge IC began tracking the new cell's discharge curve from the first full cycle.
- Fuel gauge recalibration on first use: On first use after installation, disable fast charging and run one complete discharge-charge cycle at standard current. This lets the fuel gauge IC map the new cell's discharge curve before fast charging pushes higher current into an uncalibrated cell.
Sudden shutdown at 20–30% on a replacement BM21 cell
A fresh BM21 cell has a slightly different internal resistance profile than the aged cell the fuel gauge IC was calibrated to. Under heavy load — active LTE, screen at full brightness, or a demanding app — the PMIC sees the voltage drop below the shutdown threshold even though the reported percentage still looks safe. The fuel gauge has not yet mapped where this cell's actual voltage cliff sits. One full discharge-charge cycle at standard current corrects the coulomb counter and eliminates most premature shutdowns. If shutdowns persist past three cycles, check that the battery connector is fully seated — a partial connection causes voltage sag under load.
Phone won't power on after the BM21 sat discharged in storage
Lithium-polymer cells that drop below approximately 2.5V per cell trigger a BMS lockout to prevent charging a critically depleted cell at full current. If the phone shows nothing on screen and won't respond to the charger, the BMS has entered this protective state. Connect the phone to a 5V charger and leave it untouched for 20–30 minutes — the charge IC trickle-charges the cell at low current until it climbs back above the BMS re-enable threshold, typically around 2.8–3.0V. Once the charging indicator appears on screen, the BMS has re-initialised and normal charging can resume.